The present invention relates to a paper coating composition and its production. More particularly, it relates to a coating composition (i.e. coating color) for paper comprising a mineral pigment and a certain polymer latex having improved performance.
In recent years, a high-speed printing process has been rapidly developed due to the appearance of web offset printing presses and UV irradiation curing type ink compositions. In association with such developments, various performances have been demanded of coating compositions and coated papers. Namely, coating compositions are required to have high stability to mechanical shearing force as well as significant heat resistance. Further, coated papers are required to have excellent adhesive strength, water resistance and blister resistance without generation of any unfavorable smell on irradiation of ultraviolet rays for drying. These performances of coating compositions and coated papers are much influenced by the properties of polymer latexes used as binders in the coating compositions. Particularly when the polymer latexes contain much coagula, streaks are caused on application with a blade, staining occurs on calender treatment, and blanket piling occurs on printing, etc. Because of these reasons, an attempt has been made to eliminate the coagula from the polymer latexes by filtration. However, the operation is troublesome, and the resulting polymer latexes become more expensive.
On the other hand, it is known that a close relationship is present between the crosslinking structures of the polymer latexes and the performance of the coated papers prepared by application of the coating compositions containing them. Thus, in the production of the polymer latexes, chain transfer agents such as alkylmercaptans, thiuram sulfides, carbon tetrachloride and carbon tetrabromide are introduced into the reaction system for control of the crosslinking structures, i.e. regulation of the molecular weights. However, the use of alkylmercaptans or thiuram sulfides enhances the formation of coagula in the produced polymer latexes and deteriorates the mechanical stability of the coating composition incorporated therewith. Further, an unfavorable smell is generated on irradiation with ultraviolet rays. Carbon tetrachloride or carbon tetrabromide does not have a satisfactory chain transfer effect, and therefore must be used in a relatively large amount. Because of this reason, the extent of dirt in the reactor is greatly increased, and the amount of the residue in the reactor after the polymerization becomes large.